Problem 9.3: Using the oxygen differential cell from the

From Chapter 9 Problem Set. Level 1: Conceptual Mastery.

Question

Using the oxygen differential cell from the Corrosion chapter, explain how a biofilm creates the same corrosion mechanism as a deposit – but sustains it continuously.

Answer

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A deposit creates an oxygen differential cell by physically separating aerated bulk water from the oxygen-depleted zone beneath it – the covered metal becomes anodic while the surrounding exposed metal becomes cathodic. A biofilm creates the same differential, but actively maintains it: microbial respiration continuously consumes oxygen at the metal interface, ensuring the anodic zone never re-passivates. Additionally, the biofilm generates metabolic acids that lower pH and, in the case of SRB, produces hydrogen sulfide that directly attacks the metal surface. Unlike a passive deposit, the biofilm regenerates the corrosive conditions around the clock.